Thermal Expansion Tank
A small tank that absorbs the pressure increase created when water heats and expands in a closed plumbing system, protecting the water heater and valves.
Water expands as it heats. In an open plumbing system that extra volume simply pushes back toward the city main, but a closed system, one with a pressure reducing valve, check valve, or backflow preventer at the meter, has nowhere for it to go. The pressure climbs every time the water heater fires, stressing the tank, the relief valve, faucets and supply lines. A thermal expansion tank gives that expanding water a cushion: a sealed chamber with an air bladder that compresses to absorb the surge.
The most common sign of a missing or failed expansion tank is a T&P relief valve that drips, especially right after the heater runs. Homeowners often replace the dripping valve, the drip returns, and the real culprit, unmanaged expansion, goes unaddressed. A waterlogged expansion tank, one whose bladder has failed, produces the same symptom, which is why plumbers tap the tank to hear whether the top sounds hollow with air or dead with water.
Because closed systems are increasingly the norm, most jurisdictions now require an expansion tank whenever a water heater is replaced on one. It is a small part installed on the cold supply near the heater, but skipping it shortens the life of the new tank and invites the very relief-valve drips that send people back to the plumber.
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